Metallic materials having a submicron, or nanocrystalline grain structure are of great interest due to their unique properties which include extended ductility and very high yield strengths. Much work has been done with thin films, coatings, and powders to make nanocrystalline structures, but the means of making three dimensionally large structures still remains elusive.
High energy milling is probably one of the most common ways of manufacturing metal powders having a submicron size grain structure. One problem with this approach is the powder frequently becomes heavily contaminated with microscopic particles that result from the wear of the mill, attriter or grinding media used in the process
Another technique pioneered by Purdue University and now being commercialized by Nanodynamics Inc. involves compacting machining chips created at high deformation rates. The cold work induced in the machining process results in nanocrystalline grain sizes in the chips. Like high energy milling this technique suffers contamination from the machining process and also requires the use of expensive secondary operations (Hot Isostatic Pressing, extrusion, explosive compaction, etc.) to consolidate the loose powder or chips into a bulk solid. Many times, if not carefully controlled, this secondary processing can damage the initial microstructure during consolidation.
Equi-channel angular extrusion (ECAE) is a high shear process where the metal or alloy is forced through a die changing the direction of flow. Very high strains are produced resulting in grain size refinement. However, the metal may have to be passed through the die multiple times (3-4) to produce a submicron grain size making the process work and cost intensive.
Others such as A. C. Hall, L. N. Brewer and T. J. Roemer, “Preparation of Aluminum coatings Containing Homogeneous Nanocrystalline Microstructures Using the cold Spray Process”, JTTEES 17:352-359 have shown that thin coatings made from submicron grain sized powders retain this submicron grain size when the coatings are made by cold spray. In certain instances with aluminum they have even reduced the submicron grain size.